The present invention relates to suspended concrete diaphragms for the construction of floors and roofs and, more particularly, to floors made out of discrete concrete units each of which units is sufficiently light in weight to be installed without the aid of cranes and, therefore, the diaphragms are suitable for residential and light commercial construction.
There are known systems of floors composed of precast concrete units. One drawback of such floors is that the units are so heavy that cranes are needed to move and handle each of the elongate units. In addition, there are known systems involving forms and poured concrete for forming a floor in-situ. However, poured concrete is usually too heavy for most residential construction, and too expensive due to the forming and shoring work that is needed.
In contrast to precast and poured concrete, there are known systems for constructing floors assembled out of concrete blocks where the blocks are placed in abutment with one another between adjacent joists to form a continuous surface. However, in these systems, the blocks do not act structurally as part of the horizontal slab spanning between supports; that is, the blocks only act to support localized loads, including their own weight, strictly corresponding to their area. These blocks merely act as forms, filling the spaces between tension joists and providing a surface onto which concrete can be poured. It is the poured concrete, when it cures, acting in unison with the steel-reinforced joists, which provides the structural strength for the floor. The poured concrete bears the horizontally-acting compressive force resulting from the weight of the floor spanning from one end support to the other, while the joists resist the corresponding tensile loads. As a result, extensive shoring is needed for this type of system to support the blocks and the concrete during installation, and the resulting floor is very heavy and inefficient due to the large amount of dead weight it contains. The installation of such floors is labor-intensive, and expensive, and the extensive shoring obstructs the space below. Furthermore, time is required after work on the floor is done for the concrete to cure before the temporary shoring can be removed and further construction can take place.
There is also a known concrete masonry floor system in which the concrete units are arranged in rows to contain both compressive and tensile resistance elements, and the units in each row are placed in direct contact against one another to transmit the horizontal forces resulting from the floor loads. In such a system, the concrete units are pre-compressed against one another by means of steel bars or cables inserted longitudinally through holes in the concrete units and tensioned during the row pre-assembly process, whereby the reinforcing bars exert compressive forces on the concrete units even before such forces are contributed by the floor spanning loads. A drawback with such a known system is caused by inevitable imperfections in the mating surfaces of the concrete units due to the nature of molding concrete blocks. These imperfections result in surfaces which do not mate perfectly, distanced by projecting points which experience high stress concentrations when the concrete units are placed in compression. Such systems have actually failed explosively due to failures in the units resulting from the stress concentrations. In order to avoid the stress concentrations which have caused this problem, it is necessary to precision-grind the mating faces of the blocks for a precise fit before the blocks are assembled. Accordingly, this system is very expensive to use.
Another type of suspended floor system which has supplanted most of the block systems described above uses a metal deck instead of blocks as a form between steel joists, the metal deck being lightweight, and corrugated so that little or no shoring is required. Concrete is poured onto the metal deck form together with conveniently placed reinforcing bars to resist the tensile loads acting on the slab from its spanning action between the joists. Although the relative light weight and cost of this system has allowed it to be widely used, its drawbacks include extensive labor to pump the concrete up onto the metal deck and to finish the structural slabs. While requiring elaborate protection of its exposed metal underside to prevent its collapse in case of a fire underneath. In some systems employing metal decks, the decks are more than forms; they act structurally with the concrete. such systems employ metal joists, and the decks and joists are subject to melting and collapse in a fire. These systems require elaborate protection of their exposed metal undersides to prevent their collapse in case of a fire underneath.